Investigating Nano-structured Catalysts at the Atomic scale by Field Ion Microscopy and Atom Probe Tomography - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 256

monitoring variations in adsorbate coverages and underlying surface struc-
tures. FIM and atom-probe techniques are respectively well suited for these
goals and both are capable of providing information on the nanostructure of
model catalysts. The co-existence of a considerable number of small planes
and facets, separated by steps, make field emitter tips ideal to study coupling
effects via surface diffusion. 35 However, the presence of an imaging field
may introduce some complexity that must be considered in 'operando'
studies of surface reactions. Processes like field dissociation, charge trans-
fer, etc. may lead to misinterpretation if field effects are neglected. 36,37
Withtheprobe-holemethodillustratedinFigure10.2,alocalchemical
analysis can be made during the ongoing reaction by using field pulses. 21
Their amplitude can be chosen so as to desorb all the gaseous species with
one pulse. With this procedure, a fresh surface is restored after each pulse
and kinetic studies can be undertaken by a systematic variation of the pulse
repetition rate, from 0.1 ms to seconds. Another approach is to use a static
imaging voltage and to add pulses of moderate amplitude which do not
affect significantly the surface reaction in progress. This so-called 'pulsed
field desorption mass spectrometry' (PFDMS) can therefore be considered
as a partially destructive method of analysis. Unfortunately, the absence of
commercially available designs means that these methods are currently
restricted to a limited number of laboratories worldwide.

d n 9 r 4 n g | 8

10.2.3 Atom Probe Tomography (APT)
10.2.3.1 Principles
Atom-probe tomography (APT) uses a position-sensitive detector to deter-
mine the original location of atoms field evaporated from the tip. The most
widely used position sensitive single ion detectors are currently based on a
set of crossed delay lines. 38 When a charged particle hits one line of a
conducting material, it causes a variation of the electric potential that
propagates towards the extremity of the line. The time difference between
the signal detection at the two ends of the line indicates where the charged
particle interacted with the line. Multi-hit events, i.e. when several ions hit
the detector within the time window after a single evaporation pulse, are
spatially resolved by crossing two or three delay lines. A microchannel plate
electron multiplier is mounted in front of the delay line detector. This
converts the initial ion impact into a highly localized, intense shower of
electrons, of sucient amplitude for detection by the delay line electronics
system. As illustrated on Figure 10.5, the latest generations of instruments
for APT incorporate a local counter-electrode situated in front of the tip. This
evolution allowed a considerable reduction of the flight paths and operating
voltages without sacrifice of the spectral resolution. It has been termed the
Local Electrode Atom Probe (LEAP t ) by the inventors and is commercially
available. These instruments offer considerable advantages in terms of their

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